Explanation: Rhea
is the second largest moon of
Saturn,
behind Titan,
and the largest without an atmosphere.
It is composed mostly of water ice, but has a small rocky core.
Rhea's
rotation and orbit are locked together (just like
Earth's Moon)
so that one side always faces Saturn.
A consequence of this is that one side always leads the other.
Rhea's leading surface is much more heavily cratered than its trailing
surface.
The above photograph was taken with the
Voyager 1 spacecraft in 1980.
NASA's Cassini spacecraft is currently on route to Saturn and
will arrive in 2004.